The presence of manganese in drinking water constitutes a problem for many water authorities both in Australia (references 26 and 48 referred to hereinafter in the LIST OF REFERENCES) and overseas (4,62) as a cause of manganese-related "dirty-water" in urban distribution systems. Manganese entering the distribution system accumulates as a black manganese oxide biofilm on pipe surfaces and causes consumer complaints when it sloughs off (4, 26, 48, 51, 52, 53, 62). In a chlorinated drinking water system the manganese oxide may be deposited chemically or may be accumulated by viable bacterial biofilms which develop in areas with insufficient chlorination (48, 51, 52, 53).
The manganese-related "dirty water" is not associated with any known health risk but the water is aesthetically unacceptable and causes economic losses by irreversible staining of washing, equipment, manufactured goods and swimming pools.
The problem is widespread in Australia with many cities and towns along the east coast from Cairns in North Queensland to Wyong and Woolongong in New South Wales experiencing problems. Many of these coastal towns rely on tourism as their major industry and are expected to maintain high standards for their tourist image. In 1985, the most affected consumer complaints reached as high as 870 per week (48, 51, 53).
Most water authorities aim through various water treatment strategies to reduce manganese in drinking water to the WHO and NHMRC recommended level of 0.05 mg/l (41,62). The American Water Works Association goal level is 0.01 mg/l (4).
A recent extensive study (48, 51, 52, 53) of the Gold Coast water distribution system has shown that manganese-related consumer complaints occur when manganese levels reach 0.02 mg/l and approaches 80 per week when levels rise to 0.05 mg/l. These consumer complaints are only an indication of the total number of consumers affected.
Current water treatment methods for the removal of manganese and iron from raw water supplies are destratification and oxygenation of the raw water supply (46,61) and chemical oxidation at the treatment plant followed by filtration (61). The most commonly used oxidants are KMnO.sub.4, chlorine, chlorine dioxide and ozone (61).
A survey of treatment plants by Green (24) indicates that the use of sand filters as a manganese removal reactor effectively restricts the filter loading rate to about 5 mh.sup.-1. Modern rapid sand filters are designed to operate at up to 9 mh.sup.-1 (32). It is evident, therefore, that if the economic benefits of high rate filtration are to be achieved for high manganese sources, then significant manganese removal must be achieved at treatment stages preceding filtration (32).
Manganese (II) is not removed by conventional water treatment processes such as alum flocculation unless an oxidation step is included. The most common oxidant is KMnO.sup.4 which converts Mn (II) to Mn (IV) and this colloidal precipitate is subsequently removed by filtration. There are practical difficulties with this method as the rate and extent of oxidation is dependent on factors such as the speciation of manganese, the characteristics of organics present and filter efficiency. These factors are often beyond the control of the plant operator. On occasions very little manganese is removed at worst the concentration may be higher after treatment that in the raw water.
Recently, chlorine and chlorine dioxide have been used in the dual roles of disinfection and oxidation (61).
Biological oxidation of manganese offers an alternative to chemical methods and is already being used to some extent in water treatment, but not to its full potential.
At neutral pH, manganese, unlike iron, is not oxidized by oxygen alone. The oxidation of manganese in natural and destratified oxygenated water storages is due to part of the action of manganese-oxidising microorganisms (22, 57, 55).
There exists in nature a variety of microorganisms such as bacteria and fungi which are capable of oxidising manganese (22, 25). Such organisms are ubiquitous in their distribution occurring widely in natural soil and water habitats. Some of these organisms are well adapted to an attached mode of growth.
Biological oxidation and removal of manganese has been shown to occur in rapid sand filters colonised by manganese-oxidising bacteria (6,14,15,38). In a comprehensive study (13) of sand filters om 21 treatment plants in Germany it was shown that the bacteria involved in manganese removal belong to the genera Hyphomicrobium, Leptothrix, Metallogenium, Siderocapsa and Siderocystis. These organisms appear to have a superior ability to adhere to surfaces and to withstand the shear forces associated with flowing water. These organisms are frequently to be found in association with iron-oxidising organisms such as Gallionella, Leptothrix and together they contribute to rapid removal of manganese and iron.
Such biologically active sand filters can be operated at loading rates of up to 13 mh.sup.-1 (13) and 24 mh.sup.-1 (38) and are therefore compatible with modern water treatment requirements. Disadvantages associated with using sand filters as packed-bed bioreactors include clogging and binding of particles as biomass develops (2). This results in reduced flow rates and a reduction in biofilm surface area available for contact with manganese. It has been shown that the oxidation of manganese occurs on extracellular polymeric slime on the surface of manganese-oxidising bacteria (13,23). Binding of particles causes channelling to occur in the packed-bed so that water passes through with inadequate treatment. These problems necessitate frequent periodic cleaning of filters by vigorous backwashing, which may result in the removal of active biofilm and time is necessary for the filter to re-establish its manganese removal efficiency (18). The utilisation of microorganisms in waste water and sewage treatment is well established. Their utilisation in the treatment of drinking water has not been widely exploited. Where they are used in the removal of manganese and iron (6,13,15,38), the process is poorly understood and has not been developed to the same level of technology as for waste water treatment. Little is known about the environmental conditions which control the growth and metabolic rate of manganese-oxidising bacteria. There is a body of research in the literature on the biochemical mechanism proposed for manganese oxidation. The results are frequently conflicting and very dependent on the organism studied (eg. 15,22).